Polycentricity as a process: An evolutionary morphological analysis of the Tehran Metropolitan Region (1975–2025)

As metropolitan regions worldwide undergo rapid spatial transformation, understanding whether decentralization leads to coherent polycentric development or fragmented dispersion has become a critical question for urban policy and planning. This study examines the long-term evolution of metropolitan spatial structure in the Tehran Metropolitan Region (TMR) by conceptualizing polycentricity as an evolutionary process shaped by interacting spatial forces rather than as a fixed spatial form. Drawing on five decades of Global Human Settlement Layer built-up volume data (1975–2025), urban centers are identified endogenously, and changes in metropolitan hierarchy are assessed using longitudinal rank–size analysis. The results reveal a sustained shift toward increased morphological polycentricity, but with pronounced temporal variation in its pace and underlying drivers. Polycentricity in TMR did not emerge as a direct outcome of decentralization alone. Instead, it developed during periods in which centrifugal dynamics were coupled with centralizing forces that re-anchored outward growth into secondary urban nodes—a process conceptualized here as centrifugal concentration. When decentralizing forces dominated without such organizational mechanisms, growth became spatially fragmented and polycentric restructuring stagnated. These findings expose a key limitation of stage-based urban life-cycle models: while effective in describing early center–periphery shifts, they lack the conceptual capacity to explain the internal reorganization of metropolitan peripheries. By explicitly incorporating both directional (centripetal–centrifugal) and organizational (centralizing–decentralizing) dimensions of spatial change, this study provides a more robust framework for interpreting metropolitan evolution. From a policy perspective, the results caution that decentralization strategies unaccompanied by sustained investments in secondary centers are likely to generate dispersion rather than coherent polycentric development.